Finger joints are used in a wide variety of applications to mate two shorter segments of wood into one longer piece of wood. The reasons for finger jointing may be varied. For example, one goal of finger jointing may be to remove knots and defects to then produce usable longer pieces of wood. Another reason for finger jointing may be to produce a finished component that has a greater strength characteristic than ordinary wood. A properly finger jointed length of wood may have a strength factor greater than the same piece of wood that has not been finger jointed.
The preferred tool for forming finger joints is known today as the finger joint cutter head which utilizes sets of stack knives bolted on at radially spaced intervals about the cutter head. This technology was developed in the 1920's by the present assignee of the instant invention as exemplified by U.S. Pat. No. 1,718,325 for what is know today as the finger joint cutting head. Finger jointing has evolved since the 1920's in that better adhesives, closer tolerances and machining precision have improved greatly over the years to provide joints that have become shorter and stronger. The demand for finger jointing has increased as diminishing supplies of wood tell us that it makes good sense to make longer pieces from short ones whenever possible. However, the basic design of the finger joint cutting head has remained unchanged for the most part over seventy (70) years since the 1920's in which the first finger joint cutting head was developed by the present assignee, Wisconsin Knife Works, Inc. The finger joint cutting head developed by Wisconsin Knife Works, Inc. has been well accepted by industry and has become the standard as evidenced by several companies copying the basic stacked knife design.
Finger jointing requires that the profile being cut into the wood is both consistent and close fitting. Therefore it is important to keep the individual knives of each set (sometimes referred to as a "circle bit") sharp and well aligned. Great care must be taken to be sure that the cutter angle on each individual circle bit is maintained as the incorrect angle may produce strength problems in the resulting wood finger joint. For example, cutters that are ground too sharp produce wood fingers that are concave which in turn produces a joint that will either not glue up correctly or that will be visibly loose. Cutters that are ground too blunt will produce wood fingers that are concave which in turn produces a finger joint that is very tight or difficult to assemble. The alignment of the individual circle bits as well as the alignment of each set of circle bits on the cutter head is crucial for a good fitting joint. Misalignment of the circle bits or sets of circle bits can cause significant problems. For example, if the circle bits are misaligned, the fingers cut into the wood can either be trimmed too short or too long. When the cutter head removes too much material from the end of the wood stock, the joint will be tight at the sides of the fingers before the fingers are able to reach the bottom of the joint. This results in a visible gap at the end of the fingers while the sides of the joint are in contact with each other resulting in a weaker and less aesthetic finger joint. To remedy this problem, the finger joint cutting head must be readjusted so that the fingers are longer. If the fingers are trimmed too long, too much wood material is left on the fingers and the resulting joint "bottoms out" before the sides of the fingers are in contact with each other. This also results in strength problems. Again, the finger joint cutter head must be accurately set to ensure the perfect fit.
Often, finger jointing is accomplished with two different heads, one to cut the male end and the other to cut the female end to match such that the cut wood fingers interlock while providing a substantially smooth outer surrounding surface between adjoined boards. Thus, this requires high precision in both sharpening and setting the circle bits on both of the cutter heads. The number of knives on the bit can be adjusted to accommodate different width sizes of lumber.